1. Field of the Invention
The present invention is related to an arrangement for suppressing an interfering component in an input signal, said arrangement comprises means for deriving an estimate of the interfering component, subtraction means for determining a difference signal from the input signal and the estimate of the interfering component.
The present invention is also related to a loudspeaking telephone.
2. Description of the Related Art
An arrangement according to the preamble is known from U.S. Pat. No. 5,390,250.
This arrangement has widespread applications such as acoustic echo cancellers and noise cancellers. In these application there is an input signal in which an interfering component is present.
This interfering component can e.g. be a signal received from the far end in a loudspeaking telephone system. This far end signal is reproduced by a loudspeaker and is received by a microphone together with the near end signal e.g. from a local speaker. The input signal is now a signal derived from the output signal of the microphone. Because amplifiers are included in the transmit path, the loop gain for a specific frequency may be greater than 1, resulting in oscillations. If the acoustic feedback is smaller than 1, oscillation will not occur, but after a certain delay an echo of the signal applied to the input of the send path will appear at the output of the receive path via the far-end echo path. In telephony this means that a speaker hears his own voice delayed by a specific period of time. This phenomenon is experienced as extremely annoying especially in case of long delays.
To prevent this undesired feedback, in an acoustic echo canceller a replica of the undesired component is derived from the far end signal by means of an adaptive filter. Said replica is subtracted from the input signal, in order to eliminate the undesired component signal.
In case of a noise cancelling system the undesired component is a noise signal coming from a noise source, e.g. the noise of a running engine in a car. To eliminate the noise signal, a replica of the noise signal is derived from a reference noise signal received from a reference transducer by means of an adaptive filter. Again, this replica is subtracted from the input signal.
A problem with adaptive filters is the limited ability to track fast changes of the transfer function to be reproduced. Such a fast change can occur due to the movement of a person in the room in which the echo canceller or noise canceller is used. This can result into a reduced amount of suppression of the undesired component, a problem that is not satisfactorily solved up to now.
The object of the present invention is to provide an arrangement according to the preamble in which the ability to cope with changes in the transfer function to be reproduced by the adaptive filter has been drastically been improved.
Therefore the arrangement according to the invention is characterised in that the arrangement comprises means for determining an estimate for the frequency spectrum of the interfering component, and in that the arrangement comprises a filter having a reduced transfer function for at least one frequency range in dependence on the frequency spectrum of the interfering component.
By determining the frequency spectrum of the interfering component and by the use of a filter for selectively attenuating the frequency ranges corresponding to the frequency determined, an additional suppression of the interfering component is obtained. Experiments have shown surprisingly that the use of the above mentioned filter has almost no perceptual effect on the desired component of the input signal. Also these experiments revealed that the additional suppression of the undesired component, allows very rapid changes in the transfer function to be reproduced without hardly any perceptual effect.
An embodiment of the invention is characterised in that the filter is arranged for deriving an output signal from the difference signal.
If the filter is arranged for deriving the output signal in dependence on the difference signal, the operation of the means for deriving the replica of the undesired component is fully decoupled from the filtering operation. This has as advantage that the convergence properties of the means for deriving the replica of the undesired component are independent of the additional filtering.
A further embodiment of the invention is characterised in that the means for determining an estimate for the frequency spectrum of the interfering component are arranged for deriving the estimate for the frequency spectrum of the interfering component from the estimate of the interfering component.
At the output of the means for estimating the interfering component generally a good estimate of said interfering component is present. Using this signal for the determination of the frequency spectrum, results in a reliable estimation of said frequency spectrum.
A further embodiment of the invention is characterised in that the filter has a transfer function having an absolute value dependent on the difference of the amplitude of the frequency of the frequency spectrum of the input signal and the amplitude of the frequency spectrum of the estimate of the interfering component.
An easy way of deriving the transfer function of the filter is the subtraction of the estimate of the amplitude spectrum of the undesired component from the amplitude spectrum of the input signal. This subtraction operation is normally performed in the frequency domain, but the present invention is not limited thereto.
A very simple embodiment of the invention is characterised in that the means for determining an estimate for the frequency spectrum of the interfering component are arranged for determining the frequency of at least one peak in the frequency spectrum, and in that the filter is arranged for attenuating components in a frequency range around the frequency of the at least one peak.
The frequency peak or peaks can e.g. be determined by calculation of linear prediction parameters. The filter can have a transfer function being dependent on the prediction parameters.